This invention generally relates to arrays of sensors that operate electronically. In particular, the invention relates to micromachined ultrasonic transducer (MUT) arrays. One specific application for MUTs is in medical diagnostic ultrasound imaging systems. Another specific example is for non-destructive evaluation of materials, such as castings, forgings, or pipelines, using ultrasound.
The quality or resolution of an ultrasound image is partly a function of the number of transducers that respectively constitute the transmit and receive apertures of the transducer array. Accordingly, to achieve high image quality, a large number of transducers is desirable for both two- and three-dimensional imaging applications. The ultrasound transducers are typically located in a hand-held transducer probe that is connected by a flexible cable to an electronics unit that processes the transducer signals and generates ultrasound images. The transducer probe may carry both ultrasound transmit circuitry and ultrasound receive circuitry.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. MUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave. One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. The systems resulting from such micromachining processes are typically referred to as “micro electro-mechanical systems” (MEMS). As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.The same definition of micromachining is adopted herein.        
Each cMUT has a membrane that spans a cavity that is typically evacuated. This membrane is held close to the substrate surface by an applied bias voltage. By applying an oscillatory signal to the already biased cMUT, the membrane can be made to vibrate, thus allowing it to radiate acoustical energy. Likewise, when acoustic waves are incident on the membrane the resulting vibrations can be detected as voltage changes on the cMUT. A cMUT cell is the term used to describe a single one of these “drum” structures. The cMUT cells can be very small structures. Typical cell dimensions are 25-50 microns from flat edge to flat edge in the case of a hexagonal structure. The dimensions of the cells are in many ways dictated by the designed acoustical response.
To achieve the best possible performance, cMUTs must be exposed to extremely high electrical fields. It has been shown by other researchers that cMUTs will only outperform conventional PZT transducers if they are operated at high electric fields near the collapse voltage of the cMUT. The ability of the cMUT structure to endure the high electric fields for arrays of many elements, each containing thousands of cells connected in parallel, with a distribution of collapse voltages is essential to the success of these devices. One shortfall with current cMUT designs lies in the electrode patterning on the cMUT, and the cascade of events that occur when a single cell short circuits to ground. Currently, the electrode on each cell is connected to its nearest neighbors using simply patterned “spoke” interconnects. In the event that a single cell forms a short circuit to ground, the entire element is effectively short-circuited to ground, due to this interconnection. The problem is compounded by the reduction in bias voltage that is available to other functioning cMUT elements due to the shorted elements. The reduced cMUT bias voltage degrades the performance of the cMUT. In addition, future cMUT arrays may contain thousands of elements instead of only several hundred. Thus, there exists a cascading effect whereby only a few individual cells out of thousands can render an entire array useless.
There is a need to improve the reliability and performance of a MUT array in the event that a single or multiple MUT cells form a short circuit to ground.